1. Field of the Invention
The present invention relates to a printing position alignment method in dot matrix printing, and a printing apparatus using the method.
2. Description of the Related Art
One type of printing apparatuses performing printing by forming dots on a printing medium uses a print head that moves in a predetermined direction relative to a printing medium and has, as printing elements, ink ejection openings arranged in a direction (e.g., in a direction in which a printing medium is conveyed) different from the predetermined direction. Nowadays, as for such a printing apparatus (an inkjet printing apparatus), there is a trend of increasing the number of ejection openings arranged in a print head to achieve a higher printing speed. Furthermore, increasingly widely used is a print head provided with multiple arrays of ejection openings corresponding to multiple ink colors so as to perform color printing. Particularly, the number of ink colors is increased in order to improve print quality, and not only cyan, magenta, yellow and black to reproduce a full color image but also inks in other color tone (color and density) are also increasingly used. For example, in some cases, light color inks are used to reduce a granular impression stemming from ink dots formed on a printing medium, or particular color inks such as red, blue and green are used to increase a color reproduction range.
Under the above circumstances, with the increase of the number of arrays of ejection openings formed in a print head, a misalignment of dot printing positions among arrays of ejection openings is more likely to occur due to a variation among ejection opening formation positions occurring at the time of manufacturing of a print head; a displacement of an attachment position of a print head; or the like. Further, also in a case of use of multiple print heads, a misalignment of dot printing positions may occur due to a relative position displacement among the print heads. In addition, even the same ejection openings may cause a misalignment between dot printing positions when performing printing (bi-directional printing) by reciprocal movement of the print head in both directions. When the misalignments of these dot printing positions occur as described above, print quality is deteriorated. One of heretofore-known technique for solving this problem is to perform a process of adjusting the dot printing positions by correcting the forgoing misalignments of dot printing positions (hereinafter, referred to as a registration process).
The registration process can be carries out in such a way that a certain array of ejection openings is determined as a reference array; the relative position misalignment between dots printed by the reference ejection opening array and dots printed by the other ejection opening array is obtained; and timing of ejecting inks is corrected based on the relative position misalignment. It is also possible to perform the registration process on misalignments of dot printing positions between a forward movement and a backward movement in bi-directional printing, by correcting the ejection timing in the same fashion.
The following method is cited as a method for obtaining an adjusting value to align dot printing positions. The method uses an array of ejection openings as a reference array and another array of ejection openings as an adjustment target array, and involves: printing multiple sample patterns (hereinafter, referred to as alignment patterns), while changing the ejection timing of the adjustment target array of ejection openings for each sample pattern; and then obtaining the adjusting value through a user's visual check on the sample patterns. Similarly, in a case of obtaining an adjusting value for dot print alignment in bi-directional printing, this method also involves: printing multiple alignment patterns while making the ejection timing in a backward movement differ from the ejection timing in a forward movement for each sample pattern; and providing the multiple alignment patterns to a user's visual check. In other words, the user selects a pattern in which a dot printing position is best matched, from among the multiple alignment patterns printed on a printing medium, and inputs its information to set an adjusting value for the printing apparatus.
However, this method forces a user to perform a complex operation of a visual judgment or a selection setting. In addition, improving an alignment accuracy requires an increase of the number of alignment patterns, so that the user needs to correctly judge small differences in misalignments of ink landed positions.
Therefore, in some cases, an alignment method is employed (e.g., Japanese Patent Application Laid-Open No. 10-329381 (1998)) in which a sensor is mounted on a carriage of an inkjet printing apparatus, and is caused to scan a printing medium so as to optically read alignment patterns, whereby the inkjet printing apparatus automatically determines an adjusting value.
Recently, the droplet size of ejecting ink has become smaller for improvement of image quality. Accordingly, an influence of an external disturbance on ink ejection or dot printing has become larger. The external disturbance includes, for example, a vibration occurring when a carriage with a print head mounted thereon moves, a change of the attitude of a print head in scanning due to distortion of a rail stay supporting the carriage, or waves (cockling) of a printing medium occurring when a pattern is printed on the printing medium. These external disturbances each not only act as a factor of a change in dot printing positions in printing of an alignment pattern, but also give an impact, if an automatic alignment is employed, on optical characteristics obtained by reading the alignment patterns with an optical sensor mounted on a carriage. In particular, in the case of an ink whose optical characteristic of alignment patterns is originally difficult to detect, like the light color ink described above, the optical sensor can only output data with a low S/N ratio, so that such ink is particularly susceptible to an influence of the external disturbance.
Possible countermeasures to check these external disturbances are to improve a mechanical accuracy of a printing apparatus, and to limit types of printing media for printing an alignment pattern thereon for an automatic alignment, to a type of printing medium enabling easy optical detection. However, these countermeasures are not desirable in terms of cost and usability. Therefore, it is strongly desired to determine an adjusting value with a certain degree of accuracy, even when an optically-read output value of an alignment pattern is influenced by an external disturbance.
As a prior art to meet such a demand, one disclosed in Japanese Patent Application Laid-Open No. 2006-102997 is cited. This document employs a method including: printing a pattern for abnormal detection in synchronization with alignment patterns; and correcting an output value obtained by reading an alignment pattern influenced by an external disturbance in alignment processing, or calculating an adjusting value by excluding an influenced pattern in calculating the adjusting value.
However, according to Japanese Patent Application Laid-Open No. 2006-102997, it is necessary to print the pattern for abnormal detection in addition to alignment patterns. Therefore, there are problems left that the performing of a registration process needs a long time; the printing of the pattern for abnormal detection accordingly increases an amount of ink to be consumed, and in some cases, increases an amount of printing media, i.e., requires more resources to be consumed.